Subsea wells need maintenance and inspection activities at a regular basis. Such activities can be carried out by means of tools delivered to the well via a wire or coiled tubing that extends from a floating vessel or a platform down into the well. Typical maintenance and inspection activities in the well are measurements and monitoring of well conditions, perforating, gravel packing, production stimulation and repair of a downhole completion or production tubing. Today's systems normally use a so-called riser tube, which extends from the vessel to the top of the well head, via which the wire or coiled tubing is fed down into the well. Such systems require restriction of movements between the vessel and the well head due to the configuration of the riser tube. Thereby, the use of such systems becomes difficult, time-consuming and costly.
As an alternative there has been proposed subsea systems that do not use a riser tube for feeding the coiled tubing down to the well, but let the coiled tubing run freely between the vessel and the well head by means of a so-called riserless system. In for example the U.S. Pat. No. 4,899,823 to Cobb et al. there is disclosed such a system, which is provided with a coiled tubing injector for injecting the coiled tubing into the well through the well head and a blow out preventer stack with associated equipment (well pressure barrier section) between the injector and the well head.
The system according to said US-patent is also provided with coiled tubing stripper elements and a well fluid stuffing box carried by the injector via which the coiled tubing is injected into the well head. The blow out preventer stack and the stuffing box may here mainly be considered to function as a lubricator assembly or lubricator means. The well pressure barrier section facilitates the connecting of the coiled tubing to the well head with regard taken to the fluid pressure in the well. Prior to lowering the injector down to the well head the coiled tubing is latched in the injector and preferably also positioned in the stuffing box.
For lowering and retrieval of the injector and the well barrier section, there are provided guide wires that extend from the vessel to the well head. When it is desired to retrieve the injector, it is first operated to withdraw the coiled tubing from the well head. The tubing is locked to the drive chains of the injector and lifted up to the vessel. As the injector is bulky this means that a rather large weight has to be retrieved just for the purpose of withdrawal of the coiled tubing/toolstring to the vessel. This is impractical and thus costly, in particular as multiple toolstrings are commonly used to accomplish the operation objectives.